Anion Binding Interaction Enhances the Robustness of Iodide for High-Performance Perovskite Solar Cells.

Owing to the ionic bond nature of the Pb-I bond, the iodide at the interface of perovskite polycrystalline films was easily lost during the preparation process, resulting in the formation of a large number of iodine vacancy defects. The presence of iodine vacancy defects can cause nonradiative recombination, provide a pathway for iodide migration, and be harmful to the power conversion efficiency (PCE) and stability of organic-inorganic hybrid perovskite solar cells (HPSCs). Here, in order to increase the robustness of iodides at the interface, a strategy to introduce anion binding effects was developed to stabilize the perovskite films. It was demonstrated that the N,N'-diphenylurea (DPU), characterized by high anionic binding constants and a Y-shaped structure, provides a relatively strong hydrogen bond donor site to effectively reduce the iodine loss during film preparation and inhibits iodide migration in the device working condition. As expected, the reduced iodine loss considerably improves the quality of the perovskite films and suppresses nonradiative recombination. The performance of the device after DPU modification was significantly increased, with the PCE rising from 23.65 to 25.01% with huge stability enhancement as well.

Challenges and developments for the blue perovskite nanocrystal light-emitting diodes.

Perovskite nanomaterials have been highly thought as next-generation light emitters after recent development owing to their benefits of simple synthesis, low-cost, large-area, and wide color gamut. Encouragingly, the external quantum efficiencies (EQEs) of green, red, and near-infrared perovskite light-emitting diodes (PeLEDs) have exceeded more than 20%. However, the performance of the blue PeLEDs is still lower than other analogs, which severely limits the applications of PeLEDs in future full-color displays. Herein, we have reviewed the advances in blue perovskite NCs and their applications in blue PeLEDs. Promising blue perovskite emitters and strategies for fabricating highly efficient blue PeLEDs based on perovskite NCs are investigated and highlighted. Moreover, we point out the main challenges in blue perovskite NC LEDs including low electroluminescence efficiency (EL), spectral instability, the difficulty of charge injection, and device optimization. The perspectives for the further development of blue PeLEDs are also presented.

Formation of Efficient Quasi-All-Polymer Solar Cells by Synergistic Effect of the Ternary Strategy and Solid Additives.

All-polymer solar cells (all-PSCs) have been widely studied owing to their unique mechanical flexibility and stability. However, all-PSCs have a lower efficiency than small-molecule acceptor-based PSCs. In the work, a ternary quasi-all-polymer solar cell (Q-all-PSC) using a synergy of the ternary strategy and solid additive engineering is reported. The introduction of PC71BM can not only match the energy level of the photoactive materials with an improved open circuit voltage (VOC) of the ternary devices but also enhance photon capture, which can improve short circuit current density. It is found that there is effective charge transfer between PC71BM and PY-IT, which can form an electron transport channel and promote efficient charge transport. Moreover, the introduction of PC71BM made the PM6/PY-IT/PC71BM ternary blends more crystalline while slightly reducing phase separation, resulting in a suitable domain size. Importantly, by introducing a high dielectric-constant PFBEK solid additive as the fasten matrix, the Q-all-PSC's efficiency can reach 16.42%. This method provides a new idea for future research on all-polymer solar cells.

Fabrication of CuS/Fe3O4@polypyrrole flower-like composites for excellent electromagnetic wave absorption.

Recently, electromagnetic radiation is a serious threat to equipment accuracy, military safety and human health. The combination with different materials to fabricate absorber composites with well-designed morphology is expected to ameliorate this issue. In here, CuS/Fe3O4@polypyrrole (CuS/Fe3O4@PPy) flower-like composites are constructed by the combination of hydrothermal method, solvothermal method and in-situ polymerization. CuS with flower-like structure consisting of nanosheets can provide a conductive backbone and large specific surface area. Hollow Fe3O4 microspheres play a key role in deciding magnetic loss, and electromagnetic waves can penetrate their hollow structure, result in multiple reflection and refraction. PPy coating can enhance the combined strength of composite, and effectively consume microwaves by scattering and multiple refraction in the intercalated structure. As expected, the minimum reflection loss (RLmin) of CuS/Fe3O4@PPy composites is -74.12 dB at 8.16 GHz with a thickness of 2.96 mm, and the effective absorption bandwidth (EAB) is 4.6 GHz (13.4-18.0 GHz) at 1.68 mm. The excellent electromagnetic wave absorption performances are attributed to the synergy effect of different components. This work provides a unique strategy for the structural design of flower-like microspheres in the field of electromagnetic wave absorption.

Ultrafast Response of Centimeter Scale Thin CsPbBr3 Single Crystal Film Photodetector for Optical Communication.

The photodetector (PD) is the key component to realize efficient optoelectronic conversion signal in the visible light communication (VLC) system. The response speed directly determines the bandwidth of the whole system. Metal halide perovskite is a neotype of low-cost solution processing semiconductor, with strong optical absorption, low trap density, and high carrier mobility, thus has been widely explored in photoelectric detection applications. However, previously reported perovskite polycrystalline photodetectors exhibit limited response speed due to the existence of grain boundaries. Here, an improved confined space method is developed through adjusting the heating area to control nucleation, resulting in centimeter scale fully inorganic perovskite CsPbBr3 thin single crystal films (SCFs) (<40 µm). The smooth surface and high crystallinity of CsPbBr3 SCFs render admirable exciton lifetime. The planar metal-semiconductor-metal photodetector using CsPbBr3 SCF as the photosensitive layer demonstrates a limit response time of 200/300 ns and a VLC within 100-500 kHz frequency for both 365 nm and white light, which is superior to previously reported CsPbBr3 polycrystalline film and single crystal photodetectors.

Fabrication of one-dimensional M (Co, Ni)@polyaniline nanochains with adjustable thickness for excellent microwave absorption properties.

The development of microwave absorbing materials with strong absorption capacity, wide bandwidth and light weight has always been a topic of concern. Herein, one-dimensional (1D) M (Co, Ni)@polyaniline (PANI) nanochains (NCs) with adjustable thickness have been successfully synthesized by reducing the mental ions under a parallel magnetic field, pretreating metal nanochains with KH550 and pre-oxidization of aniline monomer. It is found that Co has a more favorable absorption width for electromagnetic waves (EMW) and Ni aims at the absorption intensity. Furthermore, the effect of metal elements on adjusting impedance matching is more significant than their magnetic loss for composites. The minimum reflection loss (RLmin) of CoP2 can be up to -73.16 dB at 4.63 mm and the effective absorption bandwidth (EAB) is 4.98 GHz at 2.17 mm, while those of NiP2 are -65.06 dB at 3.88 mm and 5.02 GHz at 2.05 mm. The increase of PANI content can significantly reduce the matching thickness. And the RLmin of CoP3 and NiP3 can reach -58.72 dB at 2.32 mm and -65.96 dB at 1.59 mm, respectively. The absorption mechanism reveals that the matching thickness of the quarter-wavelength determines frequency location. And high absorption intensity is attributed to the synergistic effects of impedance matching, conduction loss, polarization loss, and magnetic loss. This work provides a theoretical basis for designing PANI or other conducting polymers coating magnetic nanochains for electromagnetic absorbing materials with strong absorption capacity, wide bandwidth and light weight.

Carrier Diffusion and Recombination Anisotropy in the MAPbI3 Single Crystal.

MAPbI3, one of the archetypical metal halide perovskites, is an exciting semiconductor for a variety of optoelectronic applications. The photoexcited charge-carrier diffusion and recombination are important metrics in optoelectronic devices. Defects in grain interiors and boundaries of MAPbI3 films cause significant nonradiative recombination energy losses. Besides defect impact, carrier diffusion and recombination anisotropy introduced by structural and electronic discrepancies related to the crystal orientation are vital topics. Here, large-sized MAPbI3 single crystals (SCs) were grown, with the (110), (112), (100), and (001) crystal planes simultaneously exposed through the adjusting ratios of PbI2 to methylammonium iodide (MAI). Such MAPbI3 SCs exhibit a weak n-type semiconductor character, and the Fermi levels of these planes were slightly different, causing a homophylic p-n junction at crystal ledges. Utilizing MAPbI3 SCs, the photoexcited carrier diffusion and recombination within the crystal planes and around the crystal ledges were investigated through time-resolved fluorescence microscope. It is revealed that both the (110) and (001) planes were facilitated to be exposed with more MAI in the growth solutions, and the photoluminescence (PL) of these planes manifesting a red-shift, longer carrier lifetime, and diffusion length compared with the (100) and (112) planes. A longer carrier diffusion length promoted photorecycling. However, excessive MAI-assisted grown MAPbI3 SCs could increase the radiative recombination. In addition, it revealed that the carrier excited within the (001) and (112) planes was inclined to diffuse toward each other and was favorable to be extracted out of the grain boundaries or crystal ledges.

Highly In-Plane Polarization-Sensitive Photodetection in CsPbBr3 Single Crystal.

The fully inorganic perovskite lead cesium bromide single crystal (CsPbBr3 SC) is considered as an excellent candidate semiconductor for photodetectors because of its superior humidity resistance, thermal stability, and light stability compared with organic-inorganic hybrid perovskites as well as its photoelectric properties such as large light absorption coefficient and ultralong carrier migration distance. In this Letter, we utilize the inverse temperature solubility of CsPbBr3 in ternary solvents to grow large-sized CsPbBr3 SCs. By the use of the (101) plane, CsPbBr3 SC-based photodetectors are fabricated, which exhibit excellent polarized light response characteristics. The photocurrent relies on the polarization angle in a sinusoidal fashion and shows strong anisotropic optoelectronic properties. The photodetection performance perpendicular to the y axis is significantly higher than that parallel to the y axis, and the dichroic ratio under 405 nm illumination at a bias voltage of 1 V reaches 2.65. The experimental results are consistent with the results of first-principles calculations.

Defect Origin of Emission in CsCu2I3 and Pressure-Induced Anomalous Enhancement.

Lead-free metal halide perovskites CsCu2X3 (X = Cl, Br, I) with a high photoluminescence quantum yield are promising materials for optoelectronic devices. However, the origin of photoluminescence (PL) emission is still under debate, and the anomalous dependence of PL on pressure is unclear. Here, we systemically study the effects of high pressure on the structural, electronic, and optical properties of CsCu2I3 using a diamond anvil cell (DAC) and first-principles calculations. We argue that the ground state structure of CsCu2I3 belongs to the pnma phase rather than the cmcm phase under ambient conditions. There is a structural phase transition from the pnma to the cmcm phase for CsCu2I3 at ∼5 GPa. The optical band gap derivative from absorption spectra increases from 3.57 to 3.62 eV within a pressure range of 0 to 4.03 GPa, and it then decreases over 4.03 GPa. There are two major PL emissions peaks at 2.11 and 2.32 eV, which are attributed to the intrinsic defect related trap states in CsCu2I3. Interestingly, there is an anomalous dependence of both PL emissions on pressure, such that PL peaks show a blueshift and the PL intensity is enhanced from 0 to ∼4 GPa, with redshifting and decreasing at pressures above ∼4 GPa. The anomalous evolution of the two PL emissions also suggests a defect origin of emissions.

Negative photoconductivity in Cs4PbBr6 single crystal.

Zero-dimensional (0D) inorganic perovskites, particularly Cs4PbBr6, have been attracting wide attention due to their excellent photoluminescence (PL) efficiency and spectral color purity. The PL origin of Cs4PbBr6 and the underlying photophysics, however, draw intense debate and remain controversial. Revealing the photo-excited carrier generation, separation, and recombination, as well as the roles that mobile ions play, is crucial and helpful to deeply understand the photo-physical property. Photoconductivity is one of the effective approaches to closely explore the photophysics. In this study, high-purity Cs4PbBr6 single crystals were grown from Cs-enriched solutions. Negative photoconductivity was first observed in Cs4PbBr6 through Au-crystal-Au photo-detectors, and the photocurrents under high illumination power are similar to those of the diode. It is considered that the built-in electric field produced by the charged excitons combined by the neutral excitons and vacancies of Br (VBr+) are responsible for the eccentric negative photoconductivity phenomena because of strong Coulomb interactions and low VBr+ formation energy in Cs4PbBr6.

van der Waals force layered multiferroic hybrid perovskite (CH3NH3)2CuCl4 single crystals.

In inorganic-organic perovskites, the three-dimensional arrangement of the organic group results in more subtle balance of charge, spin and space, thereby providing an attractive route toward new multiferroics. Here we report the existing of multiple ferroic orderings in inorganic-organic layered perovskites with relative strong hydrogen bond ordering of the organic chains intra plane. In addition, the inter plane in perovskite is stacking via van der Waals force. However, such magnetoelectric coupling properties for this compound have not been reported since it is difficult to characterize the properties in single crystals since most of the hybrid perovskites are usually deliquescent and unstable when exposed to air. To deal with these problems, we synthesized a (CH3NH3)2CuCl4 single crystal by using a simple evaporation technique, and demonstrated ferroelectric, magnetic and magneto-electric properties of (CH3NH3)2CuCl4. The internal hydrogen bonding of easily tunable organic unit combined with 3d transition-metal layers in such hybrid perovskites make (CH3NH3)2CuCl4 a multiferroic crystal with magnetoelectrical coupling and offer an new way to engineer multifunctional multiferroic.

Study on the thermal stress evolution in large scale KDP crystals during the crystal extraction process.

Transient numerical calculations were carried out to predict the evolution of temperature and thermal stress in traditionally grown large-size KDP crystals during the removal process, considering two methods that are used to accomplish the crystal extraction. The influence of the crystal size and the difference of temperature between the crystal and environment on the stresses inside the KDP crystals were also investigated in detail. Results indicate that, in both processes of isolating crystals, the highest stress transfers from the crystal periphery to the internal part from the early to the later time stage. In the case of extracting the crystal from solution directly after crystallization and exposing to air, the maximum stress at the crystal periphery is larger than that inside the crystal, and the probability of failure from the outside surface of crystals is large. In the case of retaining the solution for a time after crystallization, the maximum stress in the crystal internal region is larger than that of the crystal surface, leading to a large possibility to originate cracks in the inner region. Both increased crystal sizes and increased temperature differences between the crystals and the environment at the end of crystal growth are factors which aggravate crystal cracking. The maximum stress in crystals in the case of retaining the solution is less than that in the case of extracting the solution, which brings about a decreased likelihood of cracking. Thus, retaining solution for a period of time after the growth is completed, such as 96 h, is suggested to be adopted to accomplish successful crystal extraction.

Incorporation of Cesium Ions into MA1- xCs xPbI3 Single Crystals: Crystal Growth, Enhancement of Stability, and Optoelectronic Properties.

Organic-inorganic hybrid methylammonium lead iodide perovskite (MAPbI3) has attracted extensive attention in a series of optoelectronic devices. The photoelectric properties of the MAPbI3 single crystal have been revealed to be much better than those of it polycrystalline counterparts. However, its poor moisture and heat resistance severely limited further development. The introduction of Cs+ into polycrystalline films has shown to be an effective way to enhance its moisture resistance through a passivation effect. However, the entrance abilities of Cs+ into a MAPbI3 crystal lattice and the influence on photoelectric properties of a single crystal were not clear until now. Therefore, we attempted to grow large MA1- xCs xPbI3 single crystals to introduce Cs+ into the crystal lattice. The existence of Cs+ brought lattice shrinkage and enhanced stability of the MAPbI3 single crystal. A moderate quantity of Cs+ (2%) proved to heighten the photoelectric properties, whereas an excess quantity of Cs+ (5%) brought more shallow defects, which ultimately deteriorated the photoelectric properties.

Design Growth of MAPbI3 Single Crystal with (220) Facets Exposed and Its Superior Optoelectronic Properties.

MAPbI3 is deemed as the most prominent member in hybrid perovskites family because of its extremely optoelectronic properties. However, some issues and puzzles are still in expectation of their answers, such as stabilities, hysteresis, ferroelectricity, and so on. To bridge the distinctions between MAPbI3 single crystal and thin films, large-size single crystals are demanded. On the contrary, crystal structure anisotropy-dependent optoelectronic properties is an inevitable topic. A series of large-size MAPbI3 single crystals with (220) facets exposed were successfully grown, using high concentration solutions and large-size seed crystals to match growth rates of (100) and (220) facets. The optoelectronic properties of photocurrents, responsivity, EQE, and detectivity clearly showed significant anisotropy of optoelectronic properties in MAPbI3 single crystal. According to ion migration theory, the anisotropy of optoelectronic properties was interpreted. We hope this result will be helpful to guide oriented growth MAPbI3 thin films.

Polarization-Dependent Optoelectronic Performances in Hybrid Halide Perovskite MAPbX3 (X = Br, Cl) Single-Crystal Photodetectors.

Hybrid organic-inorganic lead halide perovskites (HOIPs) have received significant attention because of their impressive performances in the fields of solar cells and photoelectric detection. In the past five years, great efforts have been made to improve the crystallinity, reduce grain boundaries, and enhance the stabilities of perovskite films. Compared with films, HOIP single crystals possess fewer grain boundaries and stronger optoelectronic properties and can be applied in optoelectronic devices. As the most popular HOIP member, single crystals of MAPbX3 (X = Br, Cl) are deemed as important candidates for ultraviolet-visible photodetectors, in which the crystal structure anisotropy largely affects the detection performance. In this study, high-quality cubic single crystals of MAPbBr3 and MAPbCl3 were successfully grown from solutions. Taking advantages of their smooth (100) facets, planar metal-semiconductor-metal photodetectors were fabricated using Au interdigitated electrodes. The optoelectronic performances under nonpolarized and linearly polarized lights were explored. The optoelectronic performances were dependent on linearly polarized lights. Interestingly, both responsivity and external quantum efficiency were greatly enhanced under the excitation with linearly polarized lights. Moreover, the polarization-related optical absorptions and the electron densities within the (100) plane could be used to interpret different optoelectronic performances of single crystals of MAPbX3 (X = Br, Cl) under various linearly polarized lights.

Enhanced Optoelectronic Performance on the (110) Lattice Plane of an MAPbBr3 Single Crystal.

Hybrid organic-inorganic lead halide perovskites have attracted significant attention due to their impressive optoelectronic properties. MAPbX3 (MA= CH3NH3+, X= Cl, Br or I), the most popular member of this family, has been recognized as an important next-generation optoelectronic materials contender, and remarkable progress has been achieved in both thin films and single crystals. However, the lack of optimizations in energy harvest, transportation, carrier extraction, and process compatibility is hindering their future development. In this study, a triangle prism MAPbBr3 single crystal exposing (100) and (110) crystallographic planes was successfully synthesized, and the optoelectronic performances of these two lattice planes were systematically explored by employing a planar metal-semiconductor-metal (MSM) device. Compared to the device fabricated on the (100) plane, a 153.33% enhancement of responsivity was achieved under 10 µW irradiation and 10 V bias on the (110) plane. Finally, possible mechanism for such an enhancement was discussed based on the different defect migration behaviors of (100) and (110) planes.